Why do fasting and keto require more sodium than standard dietary advice?

When insulin falls — as it does during extended fasting or very-low-carb eating — the kidneys excrete significantly more sodium and water. Replacing roughly 3,000–5,000 mg of sodium per day in these states helps prevent the headaches, fatigue and dizziness often called "keto flu" or "fasting flu". This is higher than the standard population guidance of under 2,300 mg, which assumes a typical mixed diet.

How much water should I drink on a GLP-1 medication?

A common baseline is 30–35 mL of fluid per kilogram of body weight per day — about 2.5 litres for a 75 kg adult — plus extra for exercise or sweating. GLP-1 medications blunt thirst and appetite, and nausea or vomiting can accelerate fluid loss, so steady, scheduled sipping throughout the day matters more than relying on thirst.

Is it safe to supplement potassium?

Potassium is best obtained from food and low-dose electrolyte mixes. Unlike sodium, over-supplementing potassium can cause dangerous heart-rhythm problems, especially in people with kidney disease or on medications that affect potassium handling (ACE inhibitors, ARBs, potassium-sparing diuretics). Anyone in those groups should not follow generic potassium targets without clinician guidance.

Electrolyte & Hydration Calculator

Your inputs

Body weight

lbs

Your current body weight.

Units

Scenario

Fasting lowers insulin, so your kidneys flush more sodium and water — targets shift up.

Activity / sweat level

Exercise, heat, and heavy sweating add both fluid and sodium.

Daily fluid target

2.7 L / day

≈ 91 fl oz — sip steadily across the day

Sodium3,000–5,000 mg

Potassium3,500–4,700 mg

Magnesium350–420 mg

That sodium range is roughly 1.3–2.2 tsp of table salt across the whole day, including what's in food.

Yes — this is more sodium than standard dietary advice. Fasting and very-low-carb states lower insulin, which makes your kidneys excrete far more sodium than usual; GLP-1 medications blunt thirst and intake. Magnesium glycinate or citrate are the commonly used, well-absorbed forms. Potassium should come mostly from food.

How this works

Methodology reviewed July 2026

This tool estimates daily fluid and electrolyte targets from your body weight, the metabolic scenario you're in, and how much you sweat. Fluid is scaled from body weight; sodium, potassium and magnesium are read off scenario-specific ranges. The scenarios matter because when insulin falls — during extended fasting or very-low-carb eating — the kidneys excrete far more sodium and water, and GLP-1 medications blunt thirst and intake, so targets shift up from standard population advice.

Formula

Fluid (L/day) = (weight[kg] × 33 mL/kg) / 1000 + activity addition
  activity addition: low 0 L, moderate 0.5 L, high 1.0 L
Sodium (mg/day)   = scenario range + activity add (0 / 400 / 800 mg)
  fast 3,000–5,000 · GLP-1 2,000–3,500 · keto 3,000–5,000
Potassium (mg/day) = fast 3,500–4,700 · GLP-1 3,000–4,200 · keto 3,400–4,700
Magnesium (mg/day) = fast 350–420 · GLP-1 300–400 · keto 350–420
Salt equivalent uses 1 tsp table salt ≈ 2,300 mg sodium

Inputs

Body weight · Scenario (fast / GLP-1 / low-carb) · Activity or sweat level

Outputs

Daily fluid target · Sodium range · Potassium range · Magnesium range

Assumptions

You are a healthy adult with normal kidney function and no fluid-restricting condition.
Fluid scales at roughly 33 mL per kg of body weight before activity adjustments.
Fasting and very-low-carb states keep insulin low, raising renal sodium loss above standard guidance.
Most sodium and potassium comes from food and low-dose electrolyte mixes, not aggressive supplementation.

Limitations

These are general population estimates, not a personalized medical or dosing prescription.
Sweat rate and sweat sodium concentration vary widely between individuals; the flat activity add-ons are approximations.
The higher sodium ranges apply to fasting and keto states and are not appropriate for a standard mixed diet.

Safety

If you have kidney disease, heart failure, or take blood-pressure medication or diuretics, do not follow generic sodium or potassium targets — talk to your clinician first.
Potassium can be dangerous to over-supplement, especially with ACE inhibitors, ARBs, or potassium-sparing diuretics; get it mostly from food.
On a GLP-1, nausea or vomiting speeds fluid loss — sip on a schedule rather than waiting for thirst.

This calculator is for educational purposes only and is not medical advice, diagnosis, or treatment. Results are estimates based on published formulas and population averages — your individual values may differ. Nothing here is calculated on a server: everything runs in your browser and no data is stored or sent anywhere. Always consult a qualified clinician before making health, medication, or training decisions.